Primitive myxoid mesenchymal tumor of infancy (PMMTI) is a rare malignant tumor that occurs in the soft tissue in early childhood. This malignant tumor can occur in various parts of the body, such as the trunk, extremity, and head & neck area, and is characterized by aggressive local infiltration. There is no standardized treatment so far because this tumor is very rare worldwide. Still now, the principle of treatment so far is complete surgical resection. We performed a surgical treatment for a 3-year-old PMMTI patient who visited the hospital with an incidentally discovered mass in the posterior neck. If surgical treatment is selected as the treatment for PMMTI, it is thought that resection with sufficient margins during surgery will help reduce recurrence. In addition, since local recurrence is frequent and the average recurrence- free duration is short, careful short-term follow-up is necessary.